My dad used to smoke like a chimney, starting when he and my mom split up. It pissed me off - not only was he damaging his own lungs, but potentially mine, my brother's, and my cat's, as well, and when he smoked I took a lot longer to recover from even the smallest illness. Plus, I smelled like smoke, and I really hated that because people always assumed that I was the smoker. He quit cold turkey quite a while ago (during a time he was mad at me, so that he could use "the silent treatment" to avoid letting his nicotine-starved nerves blow up on me when we talked), and I don't think I've ever been prouder of him.
Now really, I don't much care whether most people smoke or not, but it really annoys me to be in the room with a smoker. I can tolerate it if we're holding a conversation, but from the moment the cig's lit to the moment it's put out all I can think is, "Just knock it off."